This invention relates generally to permanent magnets and is concerned more particularly with a method of making toroidal magnetic devices from magnetic powder material.
In powder metallurgical fabrication of magnets, such as rare earth-cobalt magnets, for example, the powder usually is aligned and compacted at room temperature to form a magnetic device having a desired configuration. After compacting, the packing density of the powder material may be about 75 percent of the theoretical maximum value, as determined by dividing the weight per unit volume by the density of the material. Subsequently, when the device is heated in an inert atmosphere at a sintering temperature associated with the material, further densification and three-dimensional shrinkage takes place. This densification and shrinkage generally is accompanied by a diffusion bonding of the powder particles to one another and a significant improvement in the magnetic properties of the device.
It is well-known that greater densification of the powder material is achieved at increasingly higher sintering temperatures and, generally, enhances the magnetic properties of the device. However, if the sintering temperature is too high, excessive grain growth occurs and leads to a dramatic loss in magnetic coercivity. Therefore, a sintering temperature is selected which will provide adequate densification of the powder material while minimizing the possibility of excessive grain growth occurring during the sintering operation.
The sintering method is used extensively for fabricating permanent magnets from powder material and has become the accepted method for producing rare earth-cobalt magnets. Therefore, it would seem that the sintering method is ideally suited for fabricating radially aligned toroidal magnets which have a wide applicability in gyroscopes, bearings, microwave tubes, and motors, for examples. The conventional method of producing these toroidal magnets involves a time consuming and costly procedure of assembling radially polarized, arcuate segments into a supporting ring structure. However, efforts to fabricate integral toroidal magnets by means of sintering powder material have not been successful.
It has been found generally that a sintered toroidal magnet made of radially aligned powder particles will develop radial cracks either during the sintering operation or during the subsequent annealing operation. The radial cracks may be due to stresses developed by nonuniform three-dimensional shrinkage of the compacted powder material during the sintering operation, or may be due to thermal expansion differences in the radial and circumferential directions of the toroid. A calculation of the latter effect for a sample heated at the sintering temperature yields an estimated strain of one percent, which is quite high for the usually brittle sintered material to withstand.
Therefore, it is advantageous and desirable to provide a method of fabricating radially aligned toroidal magnets in an efficient and reliable manner which overcomes the disadvantages of the sintering and other prior art methods.